The present invention relates to thermally stable ultra-hard materials, and more particularly to a cutting element incorporating a thermally stable ultra-hard material, and methods for forming the same. Ultra-hard materials are often used in cutting tools and rock drilling tools. Polycrystalline diamond material is one such ultra-hard material, and is known for its good wear resistance and hardness. To form polycrystalline diamond, diamond particles are sintered at high pressure and high temperature (HPHT sintering) to produce an ultra-hard polycrystalline structure. A catalyst material is added to the diamond particle mixture prior to sintering and/or infiltrates the diamond particle mixture during sintering in order to promote the intergrowth of the diamond crystals during HPHT sintering, to form the polycrystalline diamond (PCD) structure. Metals conventionally employed as the catalyst are selected from the group of solvent metal catalysts selected from Group VIII of the Periodic table, including cobalt, iron, and nickel, and combinations and alloys thereof. After HPHT sintering, the resulting PCD structure includes a network of interconnected diamond crystals or grains bonded to each other, with the catalyst material occupying the interstitial spaces or pores between the bonded diamond crystals. The diamond particle mixture may be HPHT sintered in the presence of a substrate, to form a PCD compact bonded to the substrate. The substrate may also act as a source of the metal catalyst that infiltrates into the diamond particle mixture during sintering.
The amount of catalyst material used to form the PCD body represents a compromise between desired properties of strength, toughness, and impact resistance versus hardness, wear resistance, and thermal stability. While a higher metal catalyst content increases the strength, toughness, and impact resistance of the resulting PCD body, this higher metal catalyst content also decreases the hardness and wear resistance as well as the thermal stability of the PCD body. This trade-off makes it difficult to provide PCD having desired levels of hardness, wear resistance, thermal stability, strength, impact resistance, and toughness to meet the service demands of particular applications, such as in cutting and/or wear elements used in subterranean drilling devices.
Thermal stability is desired during wear or cutting operations. Conventional PCD bodies may be vulnerable to thermal degradation when exposed to elevated temperatures during cutting and/or wear applications. This vulnerability results from the differential that exists between the thermal expansion characteristics of the metal catalyst disposed interstitially within the PCD body and the thermal expansion characteristics of the intercrystalline bonded diamond. This differential thermal expansion is known to start at temperatures as low as 400° C., and can induce thermal stresses that are detrimental to the intercrystalline bonding of diamond and that eventually result in the formation of cracks that can make the PCD structure vulnerable to failure. Accordingly, such behavior is not desirable.
Another form of thermal degradation known to exist with conventional PCD materials is one that is also related to the presence of the metal catalyst in the interstitial regions of the PCD body and the adherence of the metal catalyst to the diamond crystals. Specifically, the metal catalyst is known to cause an undesired catalyzed phase transformation in diamond (converting it to carbon monoxide, carbon dioxide, or graphite) with increasing temperature, thereby limiting the temperatures at which the PCD body may be used.
To improve the thermal stability of the PCD material, the catalyst material may be removed from the PCD body after sintering, to form thermally stable PCD. This thermally stable PCD material (referred to as TSP) is formed by first HPHT sintering diamond particles in the presence of a metal catalyst, forming a PCD body with the catalyst occupying the interstitial regions between the diamond crystals. Then, the catalyst material is removed from the PCD body, leaving a network of empty interstitial spaces between the diamond crystals. For example, one known approach is to remove a substantial portion of the catalyst material from at least a portion of the sintered PCD by subjecting the sintered PCD construction to an acid leaching process, such as that disclosed for example in U.S. Pat. No. 4,224,380. Applying the leaching process to the PCD results in a thermally stable material portion substantially free of the catalyst material. If a substrate was used during the HPHT sintering, it is generally removed from the PCD body prior to leaching.
As another way to improve the thermal stability of the PCD material, a carbonate catalyst has been used to form the PCD. The carbonate catalyst is mixed with the diamond powder prior to sintering, and promotes the growth of diamond grains during sintering. When a carbonate catalyst is used, the diamond remains stable in polycrystalline diamond form with increasing temperature, rather than being converted to carbon dioxide, carbon monoxide, or graphite. Thus the carbonate PCD is more thermally stable than PCD formed with a metal catalyst.
As another way to provide a more thermally stable ultra-hard diamond, diamond bodies with high diamond content have been provided, by reducing the amount of catalyst material. Additionally, binderless polycrystalline diamond has been formed, without the use of a catalyst material. The resulting diamond material has a uniform intercrystalline diamond microstructure, without catalyst material interspersed between the diamond crystals. As a result, the binderless diamond body does not suffer from differential thermal expansion between diamond and catalyst.
However, while thermally stable diamond with a reduced amount of catalyst has high hardness and wear-resistance at elevated temperature, it is difficult to incorporate into a cutting element for use in a drilling or cutting tool. For example, binderless diamond material may be formed in small quantities, due to the specialized processes used in its formation. Bonding these small pieces of thermally stable diamond to other components of a cutting element can cause cracking, which can lead to early failure of the cutting element.